Starting July 1, most large grocery stores in the state of California will be legally required to recycle plastic shopping bags. In Europe, even stricter anti-plastic measures are gaining traction. Retailers in Modbury, England, for example, recently committed to an outright plastic-bag ban. News reports have cited a statistic that the ubiquitous receptacles take 500 years to break down in landfills. How do we know?
Actually, we don't. Plastic bags have only been around for about 50 years, so there's no firsthand evidence of their decomposition rate. To make long-term estimates of this sort, scientists often use respirometry tests. The experimenters place a solid waste sample—like a newspaper, banana peel, or plastic bag—in a vessel containing microbe-rich compost, then aerate the mixture. Over the course of several days, microorganisms assimilate the sample bit by bit and produce carbon dioxide; the resultant CO2 level serves as an indicator of degradation.
Respirometry tests work perfectly for newspapers and banana peels. (Newspapers take two to five months to biodegrade in a compost heap; banana peels take several days.) But when scientists test generic plastic bags, nothing happens—there's no CO2 production and no decomposition. Why? The most common type of plastic shopping bag—the kind you get at supermarkets—is made of polyethylene, a man-made polymer that microorganisms don't recognize as food.
So, where does the 500-year statistic come from? Although standard polyethylene bags don't biodegrade, they do photodegrade. When exposed to ultraviolet radiation from sunlight, polyethylene's polymer chains become brittle and start to crack. This suggests that plastic bags will eventually fragment into microscopic granules. As of yet, however, scientists aren't sure how many centuries it takes for the sun to work its magic. That's why certain news sources cite a 500-year estimate while others prefer a more conservative 1,000-year lifespan. According to some plastics experts, all these figures are just another way of saying "a really, really long time."
Sometimes, even banana peels don't decompose once they reach the landfill. For sanitary reasons, modern landfills are lined on the bottom with clay and plastic to keep waste from escaping into the soil and are covered daily with a layer of earth to reduce odor. The landfill, then, acts like a trash tomb—the garbage within receives little air, water, or sunlight. This means that even readily degradable waste objects, including paper and food scraps, are more likely to mummify than decompose.
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Explainer thanks Ramani Narayan of Michigan State University and Charles Moore of the Algalita Marine Research Foundation.